Multiple Input Multiple Output (MIMO) is a term referring to multi-antenna technology employing a plurality of transmit antennas and a plurality of receive antennas. Using this technology, data transmission/reception efficiency may be improved. That is, multiple antennas may be used at the transmitter or the receiver of a wireless communication system to increase communication capacity and improve performance. In this document, MIMO can be referred to as “multiple antenna.”
Multiple antenna technology does not rely upon a single antenna path to receive an entire message. Instead, in multiple antenna technology, the message is recovered by constructively combining data fragments received by multiple antennas. With the multiple antenna technology, data rate may be improved within cell coverage of a specific size, or the system coverage may be increased with particular data rate ensured. In addition, this technology may be used in a wide range of applications including a mobile communications terminal and a repeater. With multiple antenna technology, limitations in data traffic of the conventional technology employing a single antenna for mobile communication may be overcome.
FIG. 1 is a view illustrating the configuration of a typical multiple antenna (MIMO) communication system. In this system, NT transmit antennas are installed at the transmitter and NR receive antennas are installed at the receiver. When plural antennas are used at both the transmitter and receiver, as in this case, the channel transmission capacity is theoretically increased, compared to the case in which only one of the transmitter and the receiver has a plurality of antennas. Increase in channel transmission capacity is proportional to the number of antennas. Accordingly, transfer rate is increased and frequency efficiency is improved. When the maximum transfer rate upon use of one antenna is Ro, the transfer rate upon use of multiple antennas may be theoretically increased by a product of Ro and the rate increase ratio Ri.
For example, in an MIMO communication system employing four transmit antennas and four receive antennas, it is theoretically possible to acquire a transfer rate which is four times that of a single antenna system. Since the theoretical increase in the capacity of the multiple antenna system was proved in the mid-1990s, various technologies to substantially improve a data transfer rate have been actively developed up to now, and some of theses technologies are already applied to various wireless communications standards such as the third-generation mobile communications and the next-generation wireless local area network (LAN).
The multiple antenna technology can be divided into spatial diversity and spatial multiplexing. Spatial diversity is a scheme that increases transmission reliability using data symbols transmitted through various channel paths, and the spatial multiplexing is a scheme that enhances the transfer rate by simultaneously transmitting multiple data symbols using multiple transmit antennas. In addition, when these two schemes are properly combined, advantages of both schemes may be properly obtained.
Recently, such multiple antenna technology has also been applied to terminals. Depending on mobility, terminals can be divided into mobile/portable terminals and stationary terminals. The mobile terminals can be sub-divided into handheld terminals and vehicle mounted terminals depending upon whether or not the terminals can be directly carried around by the user.
As the terminals are diversified, some of the terminals are implemented as multimedia players having such functions as capture of images or video, reproduction of music or video files, execution of games, and reception of broadcast signals. In addition, some terminals support connection to a vehicle, thereby providing a more convenient environment for voice calls when connected to the speaker or microphone of the vehicle over a short-range communication network. Meanwhile, a technology to enable reproduction of multimedia files of a terminal through an audio/video system of the vehicle is also commercialized.
In view of communication performance, however, a terminal mounted in a vehicle may undergo a problem of signal attenuation (in-car penetration loss) by about −20 dB when a signal reaches the terminal through the glass or metal sheet of the vehicle. This phenomenon may increase power consumption in the terminal and deteriorate quality of service. Moreover, in the case of a mobile terminal, due to size constraints, positioning of antennas to achieve diversity may be limited in implementing the multiple antenna technology.